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Mix Recipes for Decorative Concrete
by Cindy Grawl
In decor uses, the size and the shape of aggregates used in the mix design are important, says Malisch. Each contractor has an opinion about particle size, and many say it depends on what kind of finish will be used.
Here is the conundrum. In decor, texture and pattern are vital, and they are affected by the size aggregate used. The deeper the stamped impression is to be, the smaller the stone needed, but the smaller the stone, the more cement is needed, and thus the more water required.
According to Tom Ralston, president of Ralston Concrete, Santa Cruz, California, using a smaller pea gravel means you have to make up for the reduced solid mass with sand, increasing the need for water and thus reducing structural integrity. While the 3⁄8" inch aggregate promotes clean edges when stamping with rubber or aluminum stamps, he says, the use of a larger angular rock improves strength and the rocks have a better tendency to interlock, reducing cracking. He says the only time a larger aggregate is difficult for stamping is when the aluminum or rubber stamps are used. He suggests using 3⁄4" angular aggregate with all polyurethane texture mat stamps to help overcome some of the shrinkage problems.
Skundrick believes that with stamped applications, a smaller rock, a 3⁄8", should be used, compensating with a higher cement content and a 45% aggregate to 55% sand ratio, just the opposite from a normal mix. He advises using plenty of matrix to get the detail needed on stamped applications.
Idiart also suggests a smaller-sized aggregate be used to help enhance the stamping impression, but mixing it with a larger size to enhance durability and quality.
Round or water tumbled rock is seen only in some areas of the country, and it is considered best in exposed designs.
Admixtures are used to customize just the right mix for a specific job, climate, and site, to control the set and to add durability. Common admixture types are classified in ASTM C494 and include water reducers, accelerators, and retarders.
Water reducers act as the name suggests: they either lower water demand while maintaining slump or they can be used to increase the slump without the need for additional water. Some provide set control in addition to water reduction and act as either accelerators or retarders. Different types of water reducers impart different amounts of water reduction, typically from about 3% to 10%. High-Range water reducing admixtures or superplasticizers offer the highest level of water reducing capability, typically ranging from 12% to about 25%. Ralston notes that superplasticizers increase slump without adding water.
Accelerators as defined by ASTM C494 as “an admixture that accelerates the setting and early strength development of concrete.” They may or may not provide water reduction as well. They primarily allow concrete to maintain an even set when there is a temperature differentiation.
Using these admixtures in combination must be done carefully, as they can offset each other, even from the same manufacturer, so get advice before getting carried away. Says Ralston, “If you use admixtures indiscriminately, using too much retardant for example, it can wreak havoc, delay your setup and create a spongy effect.”
Giving the mix the opposite effect are the retarders. These admixtures slow the rate of setting and are sometimes used to offset the accelerating effect of hot weather or to delay the set, allowing for special finishing processes, which may often times increase strength. Admixtures are usually dosed in fluid ounces for every 100 pounds of cement per cubic yard of concrete.
The design must result in a durable concrete that can stand up to freeze-thaw cycles in wintry climates. This is accomplished through the use of air entrainment, either from using air-entraining cement or from air-entraining admixtures, described in ASTM C260. The microscopic voids provided by air entrainment allow space for frozen water to expand so it won’t fracture the concrete. The need for air entrainment, says Malisch, varies from region to region, running from 3% to 7% of the volume of the concrete.
Skundrick notes that air entrainment additives can make a finish sticky, but the solutions to this lie with the finishing process, not with a change in mix design.
Ralston suggests that contractors look into the use of such materials as acrylic polymers, which help increase a mix’s adherence to old concrete and improve concrete’s flexibility. Such polymers, says Ralston, mean a lower water-to-cement ratio and help reduce shrinkage as the plastic fills these interstitial voids. “As the industry develops,” he says, “modified acrylics and latex additives will become more important in mix designs as we strive for sophisticated applications. Tinkering with these admixtures will mean we will begin to see some awesome things.”
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